Optical waveguides – Planar optical waveguide – Thin film optical waveguide
Reexamination Certificate
2001-08-16
2003-09-23
Feild, Lynn D. (Department: 2839)
Optical waveguides
Planar optical waveguide
Thin film optical waveguide
C385S129000, C385S131000
Reexamination Certificate
active
06625370
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical waveguide and a fabricating (or manufacturing) method thereof, and an optical waveguide circuit, used in the fields of optical communication, optical signal processing, and optical measurement.
2. Description of the Related Art
Due to the worldwide spread of the Internet, optical communication systems using a WDM (wavelength-division multiplexing) technique or the like have spread commercially, in particular, into North America and the like. The WDM technique enables high-speed transmission of large amounts of data such as image data or video data.
Accordingly, research and development of lightwave (or optical) circuits constituting optical communication systems has been accelerated. In particular, waveguide-type lightwave circuits (i.e., optical waveguide (or wave-guiding) circuits), which can include optical waveguides formed on a single planar substrate by using the LSI fine-processing technique, have become the focus of attention because they have a high degree of integration and superior mass productivity, and accordingly, lightwave circuits having superior performance and a complicated structure can be realized using such optical waveguide circuits.
That is, optical waveguide circuits can provide various kinds of lightwave circuits by using functions of optical interference. In particular, optical wavelength-division multiplexing and demultiplexing devices are key devices in WDM systems.
FIG. 5A
shows an arrayed-waveguide grating (AWG) type optical wavelength-division multiplexing and demultiplexing device as an example of the optical waveguide circuits. This AWG type optical wavelength-division multiplexing and demultiplexing device comprises input channel waveguides
1
, output channel waveguides
2
, a channel waveguide array
3
, an input slab waveguide
4
for connecting the input channel waveguides
1
and the channel waveguide array
3
, and an output slab waveguide
5
for connecting the output channel waveguides
2
and the channel waveguide array
3
.
FIG. 5B
shows an asymmetric Mach-Zehnder interferometer (MZI) type optical attenuator as another example of the optical waveguide circuits. In this device, two input waveguides
6
, two output waveguides
7
, and two arm waveguides
8
are connected with each other via two 3-dB directional couplers
9
, and a thin-film heater type phase shifter
10
is formed on each arm waveguide
8
.
FIG. 6
is a cross-sectional view of a conventional optical waveguide. On a silicon (Si) substrate
11
, a lower cladding
12
, a core
13
, and an upper cladding
14
are formed.
Here, the polarization state of an optical signal passing through an optical network is not controlled; thus, the relevant optical waveguide circuit must have polarization-insensitive characteristics.
However, in the actual optical waveguide circuit, the core of each optical waveguide has geometrical birefringence or stress-induced birefringence, which causes polarization dependence. In particular, even if a silica-based optical waveguide circuit is employed and the core of each optical waveguide has an almost square cross-sectional shape (in this case, the geometrical birefringence can significantly be disregarded), the material and composition of the substrate generally differ from those of the waveguide portion; thus, various kinds of stress components are imposed on the core, and in most cases, stress imposed in the horizontal direction is not the same as that imposed in the vertical direction. As a result, due to photoelasticity, difference of the refractive index between the horizontal direction and the vertical direction (that is, stress-induced birefringence) occurs, thereby generating polarization dependence in the optical wave-guiding characteristics.
FIG. 3A
is a graph showing an example of the optical transmitting characteristics of an AWG type optical wavelength-division multiplexing and demultiplexing device which is fabricated using silica-based glasses.
FIG. 4A
is a graph showing an example of the optical transmitting characteristics of an asymmetric MZI type attenuator which is also fabricated using silica-based glasses. As shown in each figure, in each example, a TM mode and a TE mode indicate different optical output characteristics, that is, polarization dependence is present.
In order to resolve such polarization dependence, a method of controlling the composition of the optical waveguide circuit (Reference 1:S. Suzuki et al., “Polarization-Insensitive Arrayed-Waveguide Gratings Using Dopant-Rich Silica-Based Glass with Thermal Expansion Adjusted to Si Substrate”, Electronics Letters, Vol. 33, No. 13, pp. 1173-1174, 1997; and Reference 2:S. M. Ojha et al., “Simple Method of Fabricating Polarization-Insensitive and Very Low Crosstalk AWG Grating Devices”, Electronics Letters, Vol. 34, pp. 78-79, 1998), and a method of inserting a wave plate (Reference 3:Y. Inoue et al., “Polarization Mode Converter with Polyimide Half Waveplate in Silica-Based Planar Lightwave Circuits”, IEEE Photonics Technology Letters. Vol. 6, No. 5, pp. 175-177, 1994) are known.
The features of such conventional methods are shown in Table 1.
TABLE 1
OPTICAL
RELIABILITY
CHARACTERISTICS
PRODUCTIVITY
COST
OF DEVICE
COMPOSITION
◯
◯
◯
X (crack etc.)
CONTROL
WAVE PLATE
&Dgr; (increase of loss)
X
X
◯
INSERTION
PRESENT
◯
◯
◯
◯
INVENTION
The method of controlling the composition has superior optical characteristics, productivity, and cost effectiveness; however, when this method is employed, the reliability of the device degrades. For example, when silica-based glasses are used, the stress inside the glass changes from compressive stress to tensile stress owing to the composition control. Therefore, the glass portion of the waveguide may easily have a crack or the like. Similarly, in an optical waveguide circuit using optical waveguides made of a material other than silica-based glasses (that is, a polymer or the like), the composition for realizing the polarization insensitivity does not always agree with the composition for obtaining the reliability of the device.
Currently, the method of inserting a wave plate is the leading method because the reliability of the device does not degrade. However, the power loss of the signal increases by approximately 0.5 to 1.0 dB, and generally, it is difficult to obtain preferable productivity and cost effectiveness in this method. This is because both the process of forming a groove for inserting a wave plate and the process of inserting the wave plate must be performed for each finished chip of the optical waveguide circuit.
As explained above, no currently-known method of realizing polarization insensitivity can satisfy both (i) the required characteristics related to the device, such as the optical characteristics and reliability, and (ii) the required productivity and cost effectiveness.
SUMMARY OF THE INVENTION
In consideration of the above circumstances, an object of the present invention is to provide an optical waveguide and an optical waveguide circuit having polarization insensitivity or a required low-level polarization dependence without degradation of the optical characteristics and reliability, and to provide a fabricating method of an optical waveguide and an optical waveguide circuit having polarization insensitivity or a required low-level polarization dependence without increasing the fabricating burden and the cost.
The above and other objects, and distinctive features of the present invention will be shown by the following explanations and attached drawings.
Therefore, the present invention provides an optical waveguide comprising:
a planar substrate;
a lower cladding which is provided on the planar substrate, where the lower cladding has a ridge;
a core, provided on the ridge of the lower cladding, for transmitting light; and
an upper cladding provided in a manner such that the core is covered with the upper cladding, and wherein:
the ridge has a shape predetermined
Hida Yasuhiro
Itoh Mikitaka
Kasahara Ryoichi
Saida Takashi
Darby & Darby
Feild Lynn D.
Nguyen Son V.
Nippon Telegraph and Telephone Corporation
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